National CP3BT26 - Manual (Page 199)

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CP3
BT26
26.2.2 Mode 2: Dual Input Capture
Mode 2 is the Dual Input Capture mode, which measures
the elapsed time between occurrences of external events,
and which also provides a separate general-purpose timer/
counter.
Figure 101 is a block diagram of the Multi-Function Timer
configured to operate in Mode 2. The time base of the cap-
ture timer depends on Timer/Counter 1, which counts down
using the clock selected with the Timer/Counter 1 clock se-
lector. The TA and TB pins function as capture inputs. A
transition received on the TA pin transfers the timer contents
to the TCRA register. Similarly, a transition received on the
TB pin transfers the timer contents to the TCRB register.
Each input pin can be configured to sense either rising or
falling edges.
The TA and TB inputs can be configured to preset the
counter to FFFFh on reception of a valid capture event. In
this case, the current value of the counter is transferred to
the corresponding capture register and then the counter is
preset to FFFFh. Using this approach allows software to de-
termine the on-time and off-time and period of an external
signal with a minimum of CPU overhead.
The values captured in the TCRA register at different times
reflect the elapsed time between transitions on the TA pin.
The same is true for the TCRB register and the TB pin. The
input signal on the TA or TB pin must have a pulse width
equal to or greater than one System Clock cycle.
There are three separate interrupts associated with the cap-
ture timer, each with its own enable bit and pending bit. The
three interrupt events are reception of a transition on the TA
pin, reception of a transition on the TB pin, and underflow of
the TCNT1 counter. The enable bits for these events are
TAIEN, TBIEN, and TCIEN, respectively.
In Mode 2, Timer/Counter 2 (TCNT2) can be used as a sim-
ple system timer. The clock counts down using the clock se-
lected with the Timer/Counter 2 clock selector. It generates
an interrupt upon each underflow if the interrupt is enabled
with the TDIEN bit.
Neither Timer/Counter 1 (TCNT1) nor Timer/Counter 2
(TCNT2) can be configured to operate as an external event
counter or to operate in the pulse-accumulate mode be-
cause the TB input is used as a capture input. Attempting to
select one of these configurations will cause one or both
counters to stop.
Figure 101. Dual-Input Capture Mode
Timer/Counter 1
TCNT1
Timer 1
Clock
Capture A
TCRA
TAPND
TA
TAIEN
Timer
Interrupt 1
Preset
Underflow
TAEN
TCPND
TCIEN
Timer
Interrupt 1
Capture B
TCRB
TB
Timer/Counter 2
TnCNT2
Timer 2
Clock Underflow
TDPND
TDIEN
Timer
Interrupt 2
TBPND
TBIEN
Timer
Interrupt 1
Preset
TBEN
DS085